A quantum dot, which is an ultrafine grain having a grain size of 10 nm or less, is excellent in confinement of a carriers (electron and hole), and therefore can easily generate an exciton by electron-hole recombination. Thus, emission of light from a free exciton can be expected and light emission with high emission efficiency and a sharp emission spectrum can be achieved. Furthermore, the quantum dot can be controlled over a wide range of wavelengths using a quantum size effect, and thus attention is given to their application to light emitting devices such as semiconductor lasers and light emitting diodes (LEDs).
By the way, colloidal quantum dots is chemically synthesized in a liquid phase, and usually covered at the surface with organic molecules of a surfactant so that quantum dots do not agglomerate together. That is, the colloidal quantum dot had the disadvantage that the potential barrier is high potential barrier due to low conductivity of a surfactant caused by organic molecules, and therefore photoelectric conversion efficiency via carriers (holes and electrons) is low.
Furthermore, if a conductive polymer or a metallic material is used as a surfactant, a carrier injected into an electrode by application of a voltage passes through the surfactant from an anode to a cathode or from a cathode to an anode, thus making it difficult to efficiently confine the carrier in a quantum dot.
FIG. 9 is a schematic diagram of a photoelectric conversion device on the premise of the use of a conductive surfactant.
The photoelectric conversion device has a quantum dot layer 105 interposed between a hole transport layer 102 formed on the upper surface of an anode 101 and an electron transport layer 104 formed on the lower surface of a cathode 103. The quantum dot layer 105 is covered at the surface with a conductive surfactant 109 so that quantum dots 108 made of a core portion 106 and a shell portion 107 do not agglomerate together. That is, the quantum dot layer 105 has a laminated structure in which a large number of quantum dots 108 are provided in parallel, and the conductive surfactant 109 is interposed between quantum dots 108.
When a voltage is applied between the anode 101 and the cathode 103, a hole is injected into the anode 101 and an electron is injected into the cathode 103. As shown by arrow a and arrow b, the hole and electron as a carrier pass through the conductive surfactant 109, and the hole is transported toward the cathode 103 and the electron is transported toward the anode 101 without being entrapped in the quantum dot 108. That is, if the conductive surfactant 109 is used, the carrier is merely charged, and the carrier cannot be confined in the quantum dot 108.
Thus, techniques for confining a carrier in a quantum dot by using a surfactant having both hole-transporting and electron-transporting ligands have been researched and developed.
For example, Patent Document 1 proposes a nanograin light emitting material having a surfactant made from at least two ligands localized on the surface of the quantum dot, wherein among the ligands, at least one is a hole-transporting ligand and at least one is an electron-transporting ligand.
In a quantum-mechanical system, the state of energy possessed by a molecule corresponds to a molecular orbital where electrons exist, and can be classified into a ground state which is energetically lowest and stable and an excited state which is energetically higher than the ground state. The molecule is in a ground state before being irradiated with light, and molecular orbitals are occupied by electrons in order with the energetically lowest molecular orbital first. Among the molecular orbitals in a ground state, the highest molecular orbital is called a highest occupied molecular orbital (hereinafter referred to as “HOMO”), and the energy level corresponding to HOMO is a HOMO level. On the other hand, when irradiated with light, the molecule is brought into an excited state and molecular orbitals are brought into an empty state where they are not occupied by electrons. Among these molecular orbitals which are not occupied by electrons, the lowest molecular orbital is called a lowest unoccupied molecular orbital (hereinafter referred to as “LUMO”), and the energy level corresponding to LUMO is a LUMO level. Then, the electron moves through a transfer band and the hole moves through a valence band.
In Patent Document 1, as shown in FIG. 10, the HOMO level 122 of an electron-transporting ligand 121 is made lower than the HOMO level 124 of a hole-transporting ligand 123 and the LUMO level 125 of a hole-transporting ligand 123 is made higher than the LUMO level 126 of an electron-transporting ligand 121 to thereby improve efficiency of injecting a carrier into a quantum dot 127.
Furthermore, in Patent Document 1, as shown in FIG. 11, an electron-transporting ligand 121 is selected such that the HOMO level 122 of the electron-transporting ligand 121 is lower than the highest electron level 128 in the valence band of the quantum dot 127, whereby a hole injected into the quantum dot 127 is blocked by the electron-transporting ligand 121, and a hole-transporting ligand 123 is selected such that the LUMO level 125 of the hole-transporting ligand 123 is higher than the lowest electron level 129 in the transfer band of the quantum dot 127, whereby an electron injected into the quantum dot 127 is blocked by the hole-transporting ligand 123.
FIG. 12 is a view explaining the confinement principle of the quantum dot in Patent Document 1.
That is, the quantum dot 108 is made of a core portion 106 and a shell portion 107 covering the core portion 106, and the shell portion 107 is covered with a surfactant 133. The surfactant 133 has a hole-transporting ligand 133a and an electron-transporting ligand 133b, wherein the hole-transporting ligand 133a is localized on the hole transport layer 102 side and the electron-transporting ligand 133b is localized on the electron transport layer 104 side.
An electron from an electron transport layer 104 is easily injected into the core portion 106 since the LUMO level 136 of the hole-transporting ligand 133a is higher than the LUMO level 137 of the electron-transporting ligand 133b, while the hole-transporting ligand 133a is a barrier to an electron and the electron is confined in the core portion 106 since the LUMO level 136 of the hole-transporting ligand 133a is higher than the lowest electron level 138 in the transfer band of the core portion 106.
Furthermore, a hole from the hole transport layer 102 is easily injected into the core portion 106 since the HOMO level 139 of the electron-transporting ligand 133b is lower than the HOMO level 140 of the hole-transporting ligand 133a, while the electron-transporting ligand 133b is a barrier to a hole and the hole is confined in the core portion 106 since the HOMO level 139 of the electron-transporting ligand 133b is lower than the highest electron level 141 in the valence band of the core portion 106.
That is, the carriers (electron and hole) are confined in the quantum dot 108 by the electron block effect of the hole-transporting ligand 133a and the hole block effect of the electron-transporting ligand 133b. 
Thus, in Patent Document 1, the electron and hole are confined in the core portion 106 to thereby cause recombination of the electron-hole in the core portion 106, whereby an exciton is generated to emit light.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2008-214363 (claim 1, claims 3 to 5)